An observable electron-positron anisotropy cannot be generated by dark matter

Abstract

I demonstrate that if an anisotropy in the arrival direction of high-energy cosmic-ray electrons and positrons is observed then, barring local anisotropic diffusion, dark matter annihilation is ruled out as an explanation to the positron excess. For an observable anisotropy to originate from dark matter annihilation, the high-energy electrons and positrons must be produced in a nearby clump. I consider the annihilation pathway producing the smallest flux of gamma rays versus electrons and positrons, and the combination of clump distance and luminosity that minimizes the gamma-ray flux. I show that if an anisotropy from such a clump were detected, and if such anisotropy did not generate from anisotropic diffusion effects, then the clump would be clearly detectable as an anomalous, bright gamma-ray source with the Fermi Large Area Telescope. I also point out that the non-detection of an anisotropy is perfectly compatible with an astrophysical origin for the excess positrons that has nothing to do with dark matter.